A Massachusetts doctor and a nurse practitioner have been found not guilty in their second trial on charges they prescribed powerful painkillers to patients without legitimate medical reasons.

A federal jury in Boston acquitted former Dr. Joseph Zolot and nurse Lisa Pliner on Friday of conspiracy and drug trafficking charges. Prosecutors alleged the two improperly distributed methadone, oxycodone and fentanyl, and ignored evidence that some of their patients were misusing, abusing and even selling the drugs. Six of their patients died.

Prosecutors originally charged Zolot and Pliner with causing the deaths, but they dropped those charges last year after a Supreme Court ruling in an unrelated case established stricter standards for proving that a dealer's drugs cause a death.

Defense attorneys said Zolot and Pliner were compassionate caregivers who wrote the prescriptions in good faith to help people who were suffering from intense pain.

Zolot's attorney Howard Cooper told The Boston Globe he hopes the verdict will allow doctors to use the tools available to them to treat patients.

"Everybody recognizes that opiates are an issue in our community. What you hear a lot less about are the chronic-pain patients who need pain medication to have any quality of life and to get through the day," Cooper said.

Zolot and Pliner worked together at his Nonsurgical Orthopedic Center, which he opened in 2003 in suburban Needham.

Cooper said Zolot's business was effectively shut down after federal agents took most of his medical records in 2007. He voluntarily surrendered his medical license after state authorities began proceedings to revoke it.

Pliner gave up her license as a nurse practitioner, but continues to work as a nurse.

Her lawyer Michael Connolly told the Globe she "has tremendous compassion for the families that lost their loved ones," but doesn't believe the clinic's practices were responsible.

The defendants' first trial last year ended in a mistrial when jurors could not reach a verdict.